RPGs are more complex than most video games because RPGs themselves predate video games. They are one of the few genres that actually existed before electronic games were even invented. I'm of coarse talking about pencil and paper RPGs. You know, games like Dungeons and Dragons which use huge sets of rule books and dice with over 20 sides, etc.
Although video game RPGs are a lot more simplified than the oldschool pen and paper variety, they still inherit many of the same rules and characteristics.
When it comes right down to it, an RPG is really a game of stats and how to calculate those stats and translate them into a gaming experience. Things like, if I use this weapon combined with my character's "strength" statistic against the enemy's defense, how much damage will it inflict?
Also, if I do try to attack the enemy, what is the probability based on my "luck" statistic that it will succeed and not miss?
Or even things like, how many steps will it take to trigger a random encounter based on the last time I encountered an enemy (or maybe even based on my characters stats such as luck or speed)? When my character levels up, by what values will the rest of his/her stats by multiplied or increased by?
All stuff like that, a lot of stats and calculations mostly. Note that this is a VERY simplified explanation and I'm mostly explaining the type of math you'd have to use rather than showing it. But you get the idea.
Then there's also things like inventory systems (mostly a set of variables or an array), dialogue system (or dialogue trees, if you want a multiple choice nonlinear dialogue system), and shop system (basic math like checking if the player has enough money to buy something, or how much money to take away from his inventory after buying something, etc.)
and then of coarse everything else that goes into an RPG including basic things like story, graphics and music.
But the core of an RPG is statistics. If you're going to start an RPG, the first thing to do is set up a statistic system for your characters.
The first thing you want to set up are global variables such as Strength, Defense, Magic, Luck, Dexterity, and whatever other statistics you feel are appropriate for your game.
Then you'd design the rest of the game based around those statistics, starting with the battle system which is the main feature of any RPG. And then like I said, set up events to use and calculate those stats in order to translate them the basis of your battle system, leveling up system, and everything else.
Just a warning though, this is all much harder than I'm describing it. This is a very simplified explanation, but keep in mind that RPGs are one of the most complex types of games to make. But if you wish to try anyway, your best bet would to start with the basis of any RPG system, which are the stats and rules of the game, and how those stats are used to play the game. Again, this is all much harder than it sounds.
But in my opinion, please start out with something simpler before trying to attempt to make an RPG, especially in Construct.
But if you want to make an RPG, it's probably best to use something like RPG Maker which takes care of the whole issue of statistics, inventory, battle systems, and pretty much everything else that would normally make it pretty damn hard for you to make an RPG from scratch. Even a beginner can make a pretty decent RPG game in RPG Maker compared to what it would take to one from the ground up in Construct or even any other program for that matter.
But as for learning Construct, start with something simpler like a platform game or maybe a shoot em up. Because, to be honest, even some of the most experienced Construct users on this board probably wouldn't even try to finish making an RPG due to how complicated it is to make one. At least try to become more experienced before you even attempt it yourself.